It is known to use polymer coatings on various substrate materials. Typically, these coatings are produced using thermally activated polymerization. Heat is used to generate active centers that polymerize the coatings after a monomer has been applied to a substrate as a liquid. However, thermal polymerization requires large amounts of energy, time and expense to run high temperature ovens for extended periods of time.
It is also known to use photopolymerization to produce polymer coatings on substrates. Photopolymerization reactions are chain reactions which generate free radical or cationic active centers. In photopolymerization, energy from UV or visible light is used to polymerize the monomer. Photopolymerization has a number of advantages including savings in energy and high cure rates without the necessity of solvents.
However, photopolymerization of coatings on various substrates has not been successful because of problems with oxygen inhibition. Previously known photopolymerization systems generally have used free radical polymerization to generate free radical active centers. The free radical active centers may react with oxygen to produce unreactive peroxides and hydroperoxides. This results in a decrease in the polymerization rate and a reduction in molecular weight of the polymer. The oxygen inhibition may cause free radical polymerizations to exhibit an incomplete cure resulting in deficient coatings. A typical method for overcoming oxygen inhibition is to purge the system with nitrogen in an attempt to displace the oxygen from the monomer. Coatings having multiple layers would require nitrogen purging for each layer also requiring more than one illumination step.
Additionally, photopolymerization may also exhibit deficiencies in curing systems having pigments. Pigments may be used in a coating to provide color or to cover the surface of a substrate. Pigments may inhibit photopolymerization by directly competing with a photoinitiator absorption. Additionally, pigments may interact with light to scatter photons in multiple directions resulting in increased light attenuation for pigmented coatings. There is therefore a need in the art for an improved method of forming a layered coating that may be used in pigmented coatings. There is also a need in the art for an improved method of applying a layered coating that eliminates the need for multiple illumination steps and nitrogen purging steps.